The present invention is an electrically small, ultrawideband, high linear dynamic range active antenna for receiving energy from an electromagnetic field.
Conventional antennas primarily receive the electric field component of the electromagnetic radiation the electric field component induces a voltage in the antenna which is amplified through resonance. A conventional antenna is referred to as xe2x80x9celectrically smallxe2x80x9d if its size is less then one-quarter of the wavelength of the received electromagnetic radiation for which the antenna is tuned. The utility of these antennas is directly related to the wavelength of the electromagnetic radiation, the size of the antenna, and other known loss factors. Attempts to construct an efficient, electrically small, antenna have met with several obstacles.
Generally, the size of conventional antennas is tuned to about one-quarter of the wavelength of the electromagnetic radiation received. Typically these antennas (i.e., dipoles) have bandwidths of less than 20% of their resonant frequency for useful operation. Larger bandwidths can be obtained with the so-called xe2x80x9cfrequency independentxe2x80x9d antennas (i.e., equiangular spirals), however, even they tend to have maximum bandwidths of about 10:1 (i.e., 2-18 GHz). In such spirals, the bandwidth is set by the size of the antenna being xcex/3 at the lower end of the band and by the electrical size of the antenna feed on the high end of the band. In either case, the size of conventional antennas places at least a lower limit on the frequency of electromagnetic radiation that can efficiently be received. Also, the size of an efficient, low frequency antenna can be prohibitive for most platforms, consequently, efficiency is often sacrificed to make them smaller. For example, a conventional xcfx86 dBi antenna for detecting 1 MHz signals would be 400 feet in diameter for optimal efficiency.
Another obstacle to constructing an electrically small conventional antenna is that a reduction in the size of the antenna generally results in a corresponding reduction in its bandwidth, because of the sensitivity: wavelength correlation. Electrically small antennas must be resonant to absorb power effectively and efficiently from the incident energy. Since electrically small antennas also have a small impedance as seen at the antenna feed, additional methods for achieving resonance will be narrow band.
Still another obstacle associated with conventional passive antenna systems is the limited linear dynamic range of any preamplifiers connected to the antenna. Typically semiconductor preamplifiers have about a 100 dB linear dynamic range in the power output of the amplified signals over a 1 Hz bandwidth. In many applications this dynamic range, along with the associated sideband level increase (due to nonlinearities), is unacceptable. Quite often linear dynamic range requirements of over 130 dB are required in a 1 Hz bandwidth.
Further, since the efficiency of conventional antennas is reduced with their size, noise and other inherent losses become more important when post-processing the signal which the antenna generates when it receives the applied electro magnetic radiation. Increased inefficiency for small antennas is an unavoidable consequence of the low radiation resistance compared to resistive losses of the antenna. Still further inefficiency for small antennas can result from an impedance mismatch between the antenna impedance and the feed line impedance which is typically 50 ohms.
Superdirectivity (i.e., supergain) principles introduce additional problems. Superdirectivity refers to the ability of an electrically small antenna to have the same antenna pattern as an electrically larger antenna. Superdirectivity is typically obtained by producing a phased array of closely spaced conventional antennas. For traditional phased arrays the spacing of the elements is typically less than one half wavelength at the highest operating frequency. Consequently, the size of the antenna element will determine the phased array bandwidth. For superdirective arrays with even smaller inter-element spacing, the size of each antenna element becomes more important, because further reductions in the antenna efficiency arise from strong mutual coupling between the plurality of closely spaced antenna elements. Consequently, conventional superdirectivity (phased) arrays are inefficient and impractical.
As discussed in Welker el al., xe2x80x9cA Superconductive H-Field Antenna System,xe2x80x9d Laboratory for Physical Sciences, College Park, Md. (xe2x80x9cthe Welker articlexe2x80x9d), Welker attempted to provide an electrically small, high bandwidth antenna using superconducting quantum interference devices (SQUIDs) as the preamplifier. FIG. 2 of the Welker article provides a schematic illustration of the manner in which the SQUID preamplifier is coupled to the antenna in an attempt to improve the bandwidth and sensitivity of an electrically small antenna, but this arrangement suffers from some of the same disadvantages of conventional antenna systems. For example, the pickup loop is inherently narrow band because of its size and method of construction (i.e., the use of resistors and capacitors). Furthermore, the Welker system uses a single inefficient RF biased SQUID which in part results in a much larger pickup loop and reduced linear dynamic range.
Accordingly, it is desirable to provide a small antenna capable of wideband operation, especially an antenna that is efficient and has a large linear dynamic range. It is further desirable to provide an antenna that, instead of detecting the electric component of electromagnetic radiation, produces an output signal in response to the incident magnetic field component.
The present invention is a sensor for detecting the magnetic component of the incident electromagnetic radiation including a plurality of interconnected magnetic field transducing elements (i.e., a phased array). The array is constructed to combine the electrical energy provided by each of the elements. The antenna also includes a bias circuit, coupled to the array, for providing a bias signal to the plurality of magnetic field transducing elements. The elements may be electrical devices, such as SQUIDs, or optical devices, such as Mach-Zehnder modulators. The bias signal effectively amplifies the magnitude of the signal change resulting from a phase shift resulting from the applied magnetic field.
In one embodiment of the invention, tunnel junction elements are arranged in a multidimensional array. The antenna also includes a plurality of dummy tunnel junction elements electrically coupled to the active tunnel junction elements and positioned at the array perimeter. The dummy tunnel junction elements eliminate edge effects for the array to ensure that all the active tunnel junction elements receive substantially equal magnetic flux from the applied electromagnetic field.
In another embodiment of the invention, a magnetic flux transformer (focuser) collects the magnetic flux over a large area and distributes the flux to the plurality of magnetic field transducing elements to enhance linear dynamic range and sensitivity. A simple and compact feedback assembly is responsive to the plurality of magnetic field transducing elements for providing feedback to the magnetic flux transformer. The magnetic flux transformer is responsive to the feedback for maintaining the magnetic flux provided to the plurality of magnetic field transducing elements to achieve the desired enhancement to dynamic range. The result of these improvements is an electrically small, ultrawideband, high dynamic range sensor with high sensitivity and efficiency.